The present invention relates generally to radiation therapy planning for the treatment of tumors and is suitable for radiation therapy machines providing independent intensity modulated narrow beams of radiation. More specifically, the present invention relates to a system and method for generating intensity maps for intensity modulated therapy (IMRT) in which the effects of patient motion are taken into account.
Radiation therapy involves the treatment of tumorous tissue with high energy radiation according to a treatment plan. The treatment plan controls the radiation's placement and dose level so that the tumorous tissue receives a sufficient dose of radiation while the radiation to surrounding and adjacent non-tumorous tissue is minimal.
Intensity modulated radiation therapy treats a patient with multiple rays of radiation each of which may be independently controlled in intensity and/or energy. The rays are directed from different angles about the patient and combine to provide a desired dose pattern. Typically, the radiation source consists of either high-energy X-rays, electrons from certain linear accelerators, or gamma rays from highly focused radioisotopes such as Co60.
Methods of producing intensity modulated rays of radiation are well known in the art and include the stop and shoot method, (Xia, P., Verhey, L. J., “Multileaf Collimation Leaf Sequencing Algorithm for Intensity Modulated Beams with Multiple Static Segments,” Medical Physics, 25:1424-34 (1998)), the sliding window technique (Bortfeld, et al., “Realization and Verification of Three-Dimensional Conformal Radiotherapy With Modulated Fields,” Int'l J. Radiat. Oncol. Biol. Phys., 30:899-908 (1994)), intensity modulated arc therapy, (Yu, C. X., “Intensity-Modulated Arc Therapy With Dynamic Multileaf Collimation: An Alternative to Tomotherapy,” Physics in Medicine & Biology, 40:1435-49 (1995)), and sequential (axial) tomotherapy, (Carol, et al., “The Field-Matching Problem as it Applies to the Peacock Three Dimensional Conformal System for Intensity Modulation,” Int'l J. Radiat. Oncol. Biol. Phys., 34:183-87 (1996)).
One highly accurate IMRT method uses a planar fan beam which orbits the patient in the plane of the beam to treat a single slice of the patient at a time. Prior to reaching the patient, the fan beam is passed through a multileaf collimator (MLC) consisting of a series of opaque leaves. As the radiation source rotates around the patient, the tungsten leaves move into and out of the radiation beam modulating the intensity of individual rays of the fan beam.
An intensity value for each ray of the fan beam at each angle of the fan beam about the patient and for each slice of the patient is defined by a treatment sinogram. The treatment sinogram is prepared by a physician based on a dose map indicating the amount of radiation dose and its location throughout the patient. This type of treatment sinogram assumes that the location of the patient and the target area remains static.
However, neither the location of the patient nor the location a target area is usually static. Movement of the target area may occur based on, for example, patient breathing, digestive processes, slight variations in patient positioning, etc. Accordingly, even considering the accuracy of the IMRT method, a target area or portions of a patient area may not receive the correct dosage designated by a treatment plan.
What is needed is a system and method for the determination of intensity maps for intensity modulated radiation therapy in which the effects of patient motion are taken into account. What is further needed is such a system and method wherein an objective function is minimized using a generalized version of a linear least squares method which reduces back to the original treatment sinogram in the idealized case where there is no patient motion.